Electromechanical actuator with stator teeth dimensioned to operate a saturation bend for electrical flight controls of an aircraft

ABSTRACT

An electromechanical actuator for electrical flight controls of an aircraft, the actuator comprising a transmission shaft, four electromechanical conversion members, each having a respective stator and rotor secured to the transmission shaft, and four control systems, each dedicated to respective ones of the electromechanical conversion members. The stator has teeth and windings surrounding at least one tooth, whereas the rotor is provided with permanent magnets. Each electromechanical conversion member is a flux-concentrating member and each winding is concentric and has a single layer. The electromechanical actuator is intended in particularly for controlling a hydraulic actuator via a mechanical transmission within an electrical flight control device of an aircraft.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/767,753 filed Apr. 12, 2018, which is the U.S. National Phase of PCTApplication No. PCT/EP2016/072239 filed Sep. 20, 2016, which claimspriority to French Application No. FR 1502179 filed Oct. 16, 2015, thedisclosures of which are incorporated in their entirety by reference.

BACKGROUND OF THE INVENTION (1) Field of the Invention

The present invention relates to the field of electromechanicalactuators. It relates to an electromechanical actuator that is intendedmore particularly for use in electrical flight controls of aircraft. Thepresent invention also relates to an electrical flight control deviceincluding such an electromechanical actuator controlling a hydraulicactuator.

(2) Description of Related Art

An electromechanical actuator transforms electrical energy intomechanical energy. Electromechanical actuators are complex systemscombining various functions in order to perform this transformation. Anelectromechanical actuator is made up of an electromechanical conversionmember, means for processing control orders, electronic power means,electronic control and monitoring means, sensors in order to controlcorrectly the electromechanical conversion member, and an electricalpower supply conditioning stage.

The mechanical energy is generally delivered by an angular movement of ashaft, and it is characterized by a speed of rotation and by a torque.More particularly, that constitutes a rotary electromechanicalconversion member. The shaft may have an angular stroke limited to a fewtens of degrees. The shaft may also have an angular stroke that isunlimited so as to be capable of performing a plurality of completerotations. The electromechanical conversion member is then generallyreferred to as an “electric motor”. Under such circumstances, theelectromechanical actuator may include a mechanical transformation stagefor transforming the rotary movement of the electromechanical conversionmember into movement in translation.

Nevertheless, this mechanical energy can equally well be delivered by alinear movement of a shaft, being characterized by a speed intranslation and by a force, in which case it is more specifically alinear electromechanical conversion member that may also be referred toas an “electric slider”. The linear stroke of the shaft is limited bythe dimensions of the linear electromechanical actuator.

The electrical energy powering an electromechanical actuator, and moreprecisely its electrical power supply conditioning stage, may bedelivered by direct current (DC) or by single phase or polyphasealternating current (AC), such as three-phase AC. In addition,three-phase AC can be obtained from a DC source that is subjected toswitching or “chopping” by electronic power means controlled byelectronic return means so as to create an AC voltage, and consequentlyalternating current. The voltages powering the electric motor are phaseshifted relative to one another by 120° in order to create a three-phasepower supply.

A rotary electromechanical conversion member has at least one generallystationary stator, at least one rotor that can rotate relative to thestator and that may be located inside or outside the stator, and also anairgap between a stator and a rotor. Rotation of the rotor is generatedby interaction between two magnetic fields that are attachedrespectively to the stator and to the rotor, thereby creating magnetictorque on the rotor. Reference may then be made to a “stator magneticfield” and to a “rotor magnetic field”.

The airgap is formed by a volume of air through which magnetic fluxpasses between the rotor and the stator. The length of the airgap is acharacteristic having an influence on the performance of theelectromechanical conversion member. Specifically, it is known that thedrive torque of an electromechanical conversion member is increased byreducing the distance between the rotor and the stator, which distancecorresponds to the length of its airgap.

Nevertheless, Document FR 1 149 195 is known, which describesimprovements to electric motors used mainly in clockmaking. One of theimprovements is to increase the airgap between the rotor and the statorthus making it possible, contrary to accepted practice at that time, toincrease magnetic power even though magnetic fluxes are reduced. Thatalso makes it possible to reduce opposing fluxes due to variations inthe reluctance of the magnetic circuits. Furthermore, increasing theairgap makes fabrication easier.

The electromechanical actuator of the invention is intended particularlyfor use in electrical flight controls of an aircraft. In this particularapplication, sometimes referred to as “fly-by-wire”, an angular movementof the shaft of an electromechanical actuator is generally transformedinto movement in translation by a mechanical transmission in order tocontrol a hydraulic actuator. Consequently, the shaft of the mechanicalactuator has no need to turn through several revolutions about its axisof rotation, so its angular movement stroke can therefore be limited. Asa result, for simplification purposes in the description below, the term“electromechanical actuator” is used more simply to designate such arotary electromechanical actuator. Likewise, the term “electromechanicalconversion member” is used to designate such a rotary electromechanicalconversion member.

Document EP 2 543 589 describes primary flight controls for a main rotorand for a tail rotor of a rotary wing aircraft with an electromechanicalinterface between the electrical controls and the hydraulicservo-control actuators in order to control the amplification forcecontrolling the main rotor and/or the tail rotor. The electromechanicalinterface has an electric motor and a mechanical linkage enabling theelectric motor to control a hydraulic servo-control and consequently tocontrol the hydraulic servo-control actuators for each control axis ofthe main rotor and of the tail rotor. That system is applicable toexisting aircraft for replacing the electromechanical devices they use.Furthermore, by a mechanical linkage without any speed reduction memberbetween the electric motor and the hydraulic servo-control, the electricmotor can be shifted and off-axis, thus making it easier to integrate inthe aircraft. The speed reduction members that are conventionally usedcomprise for example reduction gearing and an epicyclic gear train.

Electromechanical conversion members in most widespread use includebrushless DC (BLDC) motors.

A brushless motor has one or more permanent magnets and its stator hasnon-permanent magnets, usually referred to as electromagnets. Theelectromagnets are generally constituted by one or more coils ofelectrical conductors wound around a ferromagnetic material and poweredwith DC. The term “winding” is used below to designate a set of one ormore such electrical conductor coils. A brushless motor generally has asensor making it possible to know the position of the rotor, togetherwith an electronic control system that switches the AC powering thestator windings. Thus, the electronic control system serves to determinethe angle and the direction of the stator magnetic field relative to therotor magnetic field, and consequently serves to cause the rotor to turnrelative to the stator.

Furthermore, within each stator winding, one or more coils may begrouped together in order to form different phases of the stator. Eachstator phase is powered by one of the phases of polyphase AC andgenerates a respective stator magnetic field. When the stator magneticfields come from the same polyphase AC, they add to form a singlerotating stator magnetic field, referred to as the stator resultant.This stator resultant then entrains rotation of the rotor field, andconsequently creates movement in rotation and torque of the rotorrelative to the stator.

AC electromechanical actuators include synchronous machines andasynchronous machines.

Synchronous machines, which include brushless motors, make use of anoffset of ninety degrees (90°) magnetic between the rotary field and thestationary field. In this way, the position of the rotor is alwaysaccurately determined. Furthermore, the rotation frequency of the rotorof a synchronous machine is proportional to the frequency of the ACpowering the stator.

Asynchronous machines have a rotor with a winding in which the coils areshort-circuited and a stator having a winding constituting non-permanentmagnets. As a result, when the winding of the stator conveys AC, itcreates a rotating stator magnetic field with the stator resultantgiving rise to the appearance of magnetic flux variation. An inducedelectromotive force (emf) appears and creates rotor currents in therotor winding. These currents give rise to torque appearing. An offsetbetween the stator field that is rotating relative to the stator and therotor field that is stationary relative to the rotary shaft of the rotorthen appears. The drawback of an asynchronous machine is the positionoffset, also referred to as “slip”, that is created between the statorfield and the rotor field. The accuracy with which the rotor shaft ispositioned is thus subject to the opposing torque applied to the shaftof the machine. In addition, the frequency of rotation of the rotor ofan asynchronous machine is not necessarily proportional to the frequencyof the AC, it being possible for a slip speed to appear between therotor and the stator magnetic field. That type of asynchronous machineis generally used for applications at constant speed and quasi-constanttorque.

Furthermore, synchronous permanent magnet brushless electric motors maybe classified in two major categories: trapezoidal flux motors andsinusoidal flux motors.

A trapezoidal flux motor possesses an emf of trapezoidal type when it isused as an electricity generator. This trapezoidal waveform of the emfis obtained by using a rotor having smooth poles, with the length of theairgap being constant. Since this airgap length does not vary, theairgap induction remains constant along a pole and reverses when themagnetic pole changes. The slope of the trapezoid is due to the distancebetween two magnetic poles.

A sinusoidal flux motor possesses emf of sinusoidal type when it is usedas an electricity generator. This sinusoidal waveform is obtained byinstalling a rotor having projecting magnetic poles of shapeapproximating a sinusoidal function. The length of the airgap is thenvariable and a major actor for the airgap induction that induces thewaveform of the emf. Thus, in order to create maximum torque whenoperating as a motor, the electricity used presents sinusoidalwaveforms. Furthermore, such a sinusoidal flux motor generally possesseslow cogging torque.

Also known is Document WO 2014/056773, which describes anelectromechanical actuator having a rotor with poles formed by permanentmagnets. The poles of the rotor include at least four zones havingdifferent magnetic properties arranged symmetrically relative to acenter of the pole. The surfaces of the permanent magnets can beconfigured so that the edge of a pole is lower than the middle of thatpole. Consequently, magnetic flux appears that is of substantiallysinusoidal distribution in the electromechanical actuator.

Likewise, Document EP 2 378 634 describes an electromechanical actuator,such as an electricity generator, having permanent magnets of sectionthat is sinusoidally shaped in a plane perpendicular to the axis ofrotation of the electromechanical actuator. Consequently, the sinusoidalshape of the permanent magnet serves to create sinusoidal airgapinduction and causes a substantially sinusoidal magnetic fluxdistribution to appear.

In addition, several winding architectures are possible for synchronousbrushless motors.

Firstly, the winding may be distributed or else concentric. With aconcentric winding, each wound tooth of the stator is surrounded by anelectrical conductor carrying a single phase of the AC powering themotor. With a distributed winding, each wound tooth of the stator issurrounded by at least two electrical conductors carrying at least twophases of the AC. A concentric winding presents safety advantages byavoiding overheating of a conductor of one phase spreading to otherphases. In contrast, a distributed winding makes it possible to create aspecific stator magnetic field as a function of the number of turns perslot, and consequently creates a sinusoidal total stator field.

Furthermore, concentric windings may be single-layer windings ordouble-layer windings. A single-layer winding makes use of only half ofthe teeth of the stator, with a winding surrounding one tooth while theadjacent teeth are left free, i.e. without any winding. This is unlike adouble-layer winding, which makes use of all of the teeth, each of whichis wound. A single-layer winding makes it possible to have slots ofsmall section for a large number of conductors. A single-layer windingalso makes it possible for windings to be spaced apart from one another,thereby limiting their interactions. In contrast, double-layer windingshave half the number of turns per tooth in order to create the sametorque as a single-layer winding. Each tooth is thus easier to wind atlower cost. However, double-layer windings give rise to a mutualinduction effect between the various phases of the machine and they runthe risk of a fault on one tooth propagating to an adjacent tooth.

Finally, it is known that the ferromagnetic materials used inelectromechanical conversion members present three distinctcharacteristics in their use as carriers of magnetic flux. These threecharacteristics are defined by a curve plotting variation in themagnetic induction of the material as a function of its coercivemagnetic field. The curve is made up of a linear first portioncharacterized by the magnetic permeability of the ferromagneticmaterial, a rounded second portion constituting a saturation bend, and alinear third portion characterized by the magnetic permeability of air.

In the great majority of situations, a ferromagnetic material is used inthe linear first portion of its curve so as to avoid any saturation ofthe material. Specifically, when the material becomes magneticallysaturated, the magnetic flux reaches a maximum value and can no longerincrease, so the torque delivered by the electromechanical conversionmember cannot increase either.

Nevertheless, Document US 2015/0054380 describes an electromechanicalactuator serving in particular for a hybrid vehicle or an electricvehicle. The electromechanical actuator has slots or holes between theteeth of its stator and/or in the proximity of each magnetic pole of itsrotor. Saturation of the magnetic flux flowing in that electromechanicalactuator then appears in those slots, serving to reduce undulations inthe torque delivered by that electromechanical actuator.

Also known are Documents DE 10 2013 112525 and US 2005/0052080, whichdescribe electromechanical actuators having a plurality ofelectromechanical conversion members for powering a vehicle.

In particular, according to Document DE 10 2013 112525, anelectromechanical actuator has two electromechanical conversion memberspowered by two control systems. Each electromechanical conversion memberis constituted by a stator co-operating with a common rotor.

Furthermore, in Document US 2005/0052080, an electromechanical actuatorhas a plurality of electromechanical conversion members that aremutually isolated in order to eliminate electrical and electromechanicalinterference. For example, an electromechanical actuator may have fiveelectromechanical conversion members.

Furthermore, Document WO 2009/082808 describes a distributed segmentedelectromechanical actuator, such as a motor-generator, having firstly aplurality of pairs of segments forming a stator, each segment beingprovided with a winding, and secondly a plurality of segments providedwith at least one permanent magnet and forming a rotor. The pair ofsegments of the stator may be connected together to form a segment groupthus constituting an electromechanical conversion member.

Finally, Document US 2011/0290581 describes an electromechanicalactuator having a stator with twelve teeth and a rotor provided with tenpoles that are separated from one another by spaces. The poles aresinusoidal in shape.

In the context of an application to flight controls, electromechanicalactuators are subjected to various operating constraints. A firstconstraint is ensuring safe operation of flight controls in the event ofa failure on an electromechanical actuator, and a second constraint isone of small volume and weight in order to be suitable for integratingin an aircraft. In order to satisfy the safety first constraint,equipment redundancy is being implemented nowadays. The core of anelectromechanical actuator, for its power portion, lies in therotor-stator torque. Nevertheless, providing redundancy for thisrotor-stator torque gives rise to extra size and to a weight penaltythat goes against the second constraint.

Redundancy is also possible for the windings of the stator, as describedin Document FR 2 493 059. That electric motor has a rotor with permanentmagnets, a stator with redundant windings, and four magnetic poles.Since the windings are redundant, the motor can operate in spite of onewinding failing. That motor can be used equally well in toys and formoving flight controls of an aircraft.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is thus to propose anelectromechanical actuator that makes it possible to overcome theabove-mentioned limitations so as to guarantee safe operation, whileoptimizing its weight and its size.

According to the invention, an electromechanical actuator comprises atransmission shaft having an axis of rotation, at least oneelectromechanical conversion member provided with a stator and a rotorsecured to the transmission shaft and rotatable about said axis ofrotation, and at least one control system. The stator is provided withteeth and windings, each winding surrounding at least one tooth, whilethe rotor is provided with permanent magnets, each having a northmagnetic pole and a south magnetic pole.

The rotor preferably rotates inside the stator so as to limit theoverall size of each electromechanical conversion member, andconsequently of the electromechanical actuator. Nevertheless, the rotorcould be situated outside the stator.

Each control system has all of the elements needed for powering and forcontrolling an electromechanical conversion member.

By way of example, each control system comprises means for processingcontrol orders, an electronic power device, an electronic control andmonitoring device, sensors, and an electrical power supply conditioningstage.

The electromechanical actuator of the invention is intended moreparticularly for electrical flight control devices of aircraft, and inparticular of rotary wing aircraft.

The electromechanical actuator of the invention is remarkable in that ithas three electromechanical conversion members and at least threecontrol systems, each control system being connected to a singlerespective electromechanical conversion member and powering theconversion member with AC.

Each control system thus powers a single conversion member with AC,which may be monophase or else polyphase.

The electromechanical actuator of the invention thus provides redundancyboth in terms of electromechanical conversion members and also in termsof control systems, each control system being dedicated to a singleelectromechanical conversion member.

The redundancy aspect is essential for ensuring continuing operation ofthe electromechanical conversion members in the event of one or more ofthe electromechanical conversion members failing. Thus, in the event ofa failure affecting one conversion member or indeed one control system,the actuator can still operate using the other electromechanicalconversion members and the other control systems that are not impactedby the failure, and that thus remain operational.

In addition, in order to reduce the overall size and weight of theelectromechanical actuator of the invention, at least twoelectromechanical conversion members may include a rotor in common. Thecommon rotor thus co-operates with the stators of these at least twoelectromechanical conversion members.

By way of example, two electromechanical conversion members powered bythree-phase AC have a rotor in common and their respective stators, eachof which has three phases, co-operate with the common rotor. Thus, twostators are assembled with the common rotor so as to form a six-phasearchitecture.

In addition, the stators co-operating with a common rotor can bemagnetically isolated by non-magnetic radial separation. Putting such astrongly non-magnetic zone in place within the stator yokes of thestators thus provides significant magnetic decoupling between thestators, thereby avoiding any mutual disturbance among them. Thisnon-magnetic radial separation may be located between two teeth of thestators in the form of a piece of the stator yokes, the stator yokessupporting in particular the teeth of the two stators. Such non-magneticradial separation leads to a reduction in the performance of theelectromechanical conversion members that is not significant.

Furthermore, in order to optimize the overall size and the weight of theelectromechanical actuator of the invention, each electromechanicalconversion member preferably includes a rotor with flux-concentratingpermanent magnets. The permanent magnets are arranged in such a manneras to provide flux concentration in the airgap, thereby maximizing thetorque of the electromechanical conversion member. Consequently, for apredetermined torque that is to be delivered by the electromechanicalconversion member, the use of flux-concentrating permanent magnets makesit possible to optimize the dimensions of the electromechanicalconversion member, and consequently to optimize its overall size andweight. The use of flux-concentrating permanent magnets may also make itpossible to use permanent magnets that are of lower performance, andconsequently that are less expensive.

Furthermore, in order to limit the risks of failures in theelectromechanical actuator of the invention, each electromechanicalconversion member preferably makes use of single-layer concentricwindings. As a result, interactions between the windings are limited.Consequently, an incident on one of the windings, such as a conductoroverheating, does not propagate to the other windings, thereby limitingthe effects of the incident.

Another cause of failures is jamming of an electromechanical conversionmember, e.g. as a result of a foreign body appearing between the rotorand the stator. The use of an airgap that is large, compared with theairgap that is normally used in equivalent electrical machines, enablesthis risk of jamming to be inhibited, thus guaranteeing continuity ofoperation of the electromechanical conversion member. The airgap of anelectromechanical conversion member is preferably greater than 1millimeter (mm). For example, the airgap may lie in the range 1 mm to 2mm.

Advantageously, the use of a large airgap also makes it possible toreduce interfering torques, such as a cogging torque or a reluctancetorque, which could give rise to disturbances in the torque developed bythe electromechanical conversion member. In addition, the use of a largeairgap makes it possible to slacken fabricating tolerances on thecomponents of the rotor and the stator, thereby leading to a drop in thecost of fabricating those components.

In addition, the magnets poles of each electromechanical conversionmember may be fractioned or else they may be integral. The use offractional magnetic poles enables the cogging torque present in theelectromechanical conversion member to be reduced, but at the cost ofcreating disturbances, in particular in the power supply current. Theuse of integral magnetic poles serves on the contrary to reduce theappearance of such disturbances.

An electromechanical conversion member has fractional poles when theratio

$\frac{N_{d} \cdot N_{ph}}{N_{p}}$is not an integer, where N_(p) is a first total number giving the numberof pairs of magnetic poles, N_(d) is a second total number giving thenumber of teeth, and N_(ph) is the number of phases of the AC poweringthe electromechanical conversion member.

In contrast, an electromechanical conversion member has integralmagnetic poles when the ratio

$\frac{N_{d} \cdot N_{ph}}{N_{p}}$is equal to an integer.

It is possible to refer to fractional winding machines forelectromechanical conversion members having fractional magnetic poles,and integral winding machines for electromechanical conversion membershaving integral magnetic poles.

Furthermore, the magnetic poles of the rotor may project and present ashape that is sinusoidal so that sinusoidal magnetic flux flows in eachelectromechanical conversion member.

In the specific circumstance of a short-circuit type failure in awinding, a large opposing torque referred to as a “short-circuit torque”appears in the electromechanical conversion member that has sufferedthis fault, and continues until the power supply to theelectromechanical conversion member is interrupted. This short-circuittorque thus prevents the rotor of that electromechanical conversionmember from rotating. Declutching devices can be used to separate thefailed electromechanical conversion member from the otherelectromechanical conversion members of the electromechanical actuatorso as to avoid the electromechanical actuator as a whole being blocked,and consequently so as to avoid losing a function.

However, the use of such declutching devices within an electromechanicalactuator then increases the overall size and weight of theelectromechanical actuator.

In order to avoid using declutching devices within an electromechanicalactuator of the invention, the teeth of each stator are dimensioned insuch a manner that each electromechanical conversion member operatesclose to its saturation zone. In particular, each electromechanicalconversion member preferably operates at the saturation bend in thecurve plotting variation of magnetic induction in the ferromagneticmaterial constituting the teeth.

Specifically, an electromechanical conversion member generally operatesfar from its saturation zone. Thus, the magnetic flux flowing in eachtooth of the stator and in the magnetic poles of the rotor can increase,thereby enabling the torque that is delivered by that electromechanicalconversion member to be increased. However, in the event of a shortcircuit in a winding of that electromechanical conversion member, theshort-circuit torque resulting from that short circuit is much greaterthan the nominal torque of the electromechanical conversion member.

In contrast, for each electromechanical conversion member of theelectromechanical actuator of the invention operating close to itssaturation zone, the nominal torque that is delivered cannot increasevery much, with the magnetic flux rapidly flowing at its maximum as soonas saturation occurs in the rotor and/or the stator. Consequently, theshort-circuit torque of an electromechanical conversion member sufferinga short-circuit type failure is then only a little greater than thenominal torque of that electromechanical conversion member. By way ofexample, the short-circuit torque of an electromechanical conversionmember may be 125% of its nominal torque.

Advantageously, two electromechanical conversion members then serve tomitigate a short-circuit type failure of one electromechanicalconversion member, with one of the two electromechanical conversionmembers compensating the majority or indeed all of the short-circuittorque coming from the failed electromechanical conversion member, whilethe other member delivers the nominal torque needed to operate theelectromechanical actuator.

Such operation of each electromechanical conversion member thus makes itpossible to maximize use of the induction of the ferromagnetic materialconstituting the teeth and the stator yoke and/or of the ferromagneticmaterial constituting the magnetic poles of the rotor. Theseferromagnetic materials are then used over the entire linear firstportion and in the saturation bend of their curve plotting variation inmagnetic induction. A maximum value of the induction, situated in thesaturation bend, is determined so as to avoid reaching total saturationof the ferromagnetic material.

The dimensioning of each electromechanical conversion member can bedetermined from the values desired for induction in key locations of thestator, in the teeth and in the stator yoke, or indeed in the rotor,with these desired induction values being as close as possible to thesaturation bend of the curve plotting magnetic induction variation.

Thus, the dimensions of the stator can be defined so that saturationappears in the stator. For this purpose, the dimensions of the statorare smaller than in an electromechanical conversion member that operatesconventionally, thereby advantageously reducing the overall size and theweight of each electromechanical conversion member.

Likewise, the dimensions of the rotor can also be defined so thatsaturation appears in the rotor, mainly for a flux-concentrating rotor,or indeed for a buried-magnet rotor.

Consequently, the major dimensions for the stator are the dimensions ofthe teeth or indeed the dimensions of the stator yoke so that saturationappears respectively in the teeth or else in the stator yoke. The majordimensions of the rotor for optimizing are the dimensions of themagnetic poles or else the dimensions of the armature of the rotor sothat saturation appears respectively in the magnetic poles or else inthe armature of the rotor.

For example, when the stator yoke is made of a stack of laminations, thewidth

_(d) of the teeth and the height h_(cs) of the stator yoke are definedby the following formulas:

$l_{d} = {{\frac{B_{g} \cdot T_{d}}{B_{\max} \cdot K_{fe}}\mspace{14mu}{and}\mspace{14mu} h_{cs}} = \frac{\Phi_{g}}{B_{\max} \cdot L_{st} \cdot K_{st}}}$where T_(d) is the axial length of a tooth expressed in meters (m),B_(g) is the induction of the airgap in nominal operation expressed inteslas (T), K_(fe) is a swelling coefficient, B_(max) is the maximumvalue of the airgap induction expressed in teslas (T), Φ_(g) is theairgap flux at a rotor pole in nominal operation expressed in webers(Wb), L_(st) is the axial length of the electromechanical conversionmember expressed in meters, and K_(st) is a stacking factor.

The swelling coefficient is a coefficient taking account of the spacebetween the laminations with which the stator is fabricated andaccommodates uncertainly about the complete length of theelectromechanical conversion member resulting from fabrication accuracy.The stacking factor serves to take account of variation in the area ofthe set of laminations for calculating magnetic fluxes.

By way of example, when the ferromagnetic material used for fabricatingthe stator yoke and the teeth is an iron-silicon alloy, the maximumvalue B_(max) of said induction is equal to 2.1 T. Thus, in a preferredembodiment of the invention, the width

_(d) of the teeth is equal to 4.4 mm and their height h_(cs) is equal to4 mm for an axial length T_(d) of a tooth of 8.8 mm, airgap inductionB_(g) of 1.09 T, a swelling coefficient K_(fe) of 1.04, an airgap fluxΦ_(g) of 0.000248 Wb, and an axial length L_(st) of theelectromechanical conversion member of 33 mm, with a stacking factorK_(st) of 0.9.

It is also possible to determine the first total number N_(p) of pairsof magnetic poles of the rotor and the second total number N_(d) ofteeth of the stator that make it possible firstly to satisfy theconstraints for fabricating such an electromechanical conversion memberand secondly to optimize the operation of the electromechanicalconversion member.

This calculation is performed for a preferred embodiment of anelectromechanical actuator of the invention in which eachelectromechanical conversion member is a three-phase synchronousbrushless motor having permanent magnets and flux concentration, with afractional pole, single-layer concentric windings, and periodicity ofsecond order.

This preferred embodiment relies on the following assumptions.

Firstly, each winding is a single-layer winding. Specifically, sinceevery other tooth is not wound, each electromechanical conversion memberhas at least two teeth per phase of the AC power supply or some numberof teeth proportional to twice the number of phases, such thatN_(d)=2·q·N_(ph), where q is a positive integer.

Since the power supply is three phase, N_(ph)=3 and consequently thesecond total number N_(d) of the teeth is such that N_(d)=2.3·q=6·q.

Furthermore, each winding is a single-layer concentric winding, whichmeans that the integer closest to the ratio

$\frac{N_{d}}{2 \cdot N_{p}},$e.g. referred to as

${round}\mspace{14mu}\left( \frac{N_{d}}{2 \cdot N_{p}} \right)$is an odd number. Specifically, each winding surrounds one toothsituated between an even slot and an odd slot, where a slot is the spacesituated between two adjacent teeth.

Each electromechanical conversion member has fractional poles, whichcorresponds to a ratio

$\frac{N_{d} \cdot N_{ph}}{N_{p}}$that is not an integer, and thus

$\frac{N_{d} \cdot N_{ph}}{N_{p}} \notin {N.}$

Each electromechanical conversion member has periodicity of secondorder.

The periodicity of a machine serves to break down the machine into anelementary machine and enables its designer to study the elementarymachine in simple manner, and then to apply the periodicity. For anelectric machine, the magnetic and electrical circuit diagrams of themachine repeat with the periodicity, so the first total number N_(p) forthe pairs of magnetic poles of the rotor and the second total numberN_(d) for the teeth of the stator are linked by the order of thisperiodicity.

For an order of two, it is possible to study half of eachelectromechanical conversion member relative to an axis of symmetry. Itis deduced therefrom that the greatest common divisor to the firstnumber of poles and the second number of teeth is this order ofperiodicity, such that

$G\; C\; D{{\left( {\frac{N_{d}}{2},\ N_{p}} \right) = 2}.}$

It can be deduced that N_(d)=4·x and N_(p)=2·y, where x and y arepositive integers. These equations mean that it is possible to find twoaxes of symmetry for each electromechanical conversion member, which canthus be split into four elementary machines.

It is also assumed that the first total number N_(p) for pairs ofmagnetic poles of the rotor is less than the second total number N_(d)for teeth of the stator, and that the first total number N_(p) for pairsof magnetic poles of the rotor is less than or equal to 20. Theseassumptions can then be expressed as follows: N_(p)<N_(d) and N_(p)≤20

It is then assumed that q=2·z for z being a positive integer, whichrepresents the fractional aspect of each electromechanical conversionmember that stems from the number of teeth per pole and per phase

$\frac{N_{d} \cdot N_{ph}}{N_{p}}$not being an integer, so the following can be written N_(d)=2.2.3·z,from which it can be deduced that:

$\frac{N_{d} \cdot N_{ph}}{N_{p}} = {\frac{2 \cdot 2 \cdot 3 \cdot 3 \cdot z}{2 \cdot y} \notin {N.}}$

Also knowing that N_(d)=4·x=2.3·q, it is possible to write:

${z = \frac{x}{3}}.$

From this it can be deduced that y∉{1; 2; 3; 6; 9; 18; z; 2·z; 3·z; 6·z;9·z; 18·z}, N_(p)∉{2; 4; 6; 12; 18; 2·z; 4·z; 6·z; 18·z} andN_(d)∉{N_(p); 2·N_(p); 3·N_(p); 6·N_(p); 9·N_(p)}.

It can then be concluded that N_(p)∈{8; 10; 14; 16; 20} and that N_(d)is a multiple of 12, for a three-phase power supply.

Furthermore, it is known that the integer closest to the ratio

$\frac{N_{d}}{2 \cdot N_{p}}$is an odd number.

Writing (2n−1) as being an odd number for any positive integer n, andthe difference:

$\frac{N_{d}}{2 \cdot N_{p}} - \left( {{2 \cdot n} - 1} \right)$then lies in a range extending from −½ to ½.

The following can then be written:

${{- \frac{1}{2}} < {\frac{N_{d}}{2 \cdot N_{p}} - \left( {{2 \cdot n} - 1} \right)} < \frac{1}{2}},$from which it can be concluded that (4n−3)·N_(p)<N_(d)<(4n−1)·N_(p).

Consequently, the first total number N for pairs of magnetic poles ofthe rotor must firstly belong to the range {10; 14; 16; 20} andsimultaneously the second total number N_(d) for the teeth of the statorcan be obtained by solving the inequality(4n−3)·N_(p)<N_(d)<(4n−1)·N_(p), where n is a positive integer and thesecond total number N_(d) for the teeth is a multiple of 12.

There follows a summary table based on the assumptions made, wherecrosses correspond to pairs formed by the first total number N_(p) forpairs of magnetic poles and the second total number N_(d) for teeth thatsatisfy the above-mentioned conditions.

N_(p) 8 10 14 16 20 N_(d) 12 x x — — — 24 — x x x x 36 — — x x x 48 — —— — x 60 — — — — — 72 — — x — — 84 x — — — — 96 — x x — —

In this application, it is the preferable to select the first totalnumber N_(p) for pairs of magnetic poles to be equal to 10 and thesecond total number N_(d) for stator teeth to be equal to 24.

With a different assumption, each electromechanical conversion memberhas integrated poles with single-layer concentric winding, which thengives rise to a ratio

$\frac{N_{d} \cdot N_{ph}}{N_{p}}$that is equal to an integer, and thus

$\frac{N_{d} \cdot N_{ph}}{N_{p}} \in {N.}$

It can then be deduced that N_(p)∈{2; 4; 6; 8; 10; 12; 14; 16; 18; 20}.The following summary table then applies for pairs formed by a firsttotal number N_(p) for pairs of magnetic poles and a second total numberN_(d) for teeth.

N_(p) 2 4 6 8 10 12 14 16 18 20 N_(d) 12 x — x — — — — — — — 24 — x — —— x — — x — 36 x — x — — — — — x — 48 — — — x — — — — x — 60 x — x — x —— — — — 72 — x — — — x — — — — 84 x — x — — — x — — — 96 — — — — — — — xx —

Furthermore, such an inequality can be determined for other embodimentsof an electromechanical actuator of the invention by changing some ofthe assumptions.

For example, for a three-phase machine having two layers of integral orfractional poles, the following inequality is obtained:

${\left( {{\frac{4}{3}n} - 1} \right) \cdot N_{p}} < N_{d} < {\left( {{\frac{4}{3}n} - \frac{2}{6}} \right) \cdot N_{p}}$which must be satisfied by the pairs formed by the first total numberN_(p) for pairs of magnetic poles and the second total number N_(d) forteeth.

In analogous manner, for a single-layer two-phase machine havingintegral or fractional poles, the inequality becomes:(2n−3)·N _(p) <N _(d)<(2n−1)·N _(p)

The present invention also provides an electrical flight control devicefor an aircraft, e.g. a rotary wing aircraft, having anelectromechanical actuator, a hydraulic actuator, and a mechanicaltransmission for each control axis of the aircraft.

Each electromechanical actuator serves to transform an electrical ordercoming from a flight control device of the aircraft into a mechanicalorder. This low power mechanical order is transmitted via a mechanicaltransmission to the hydraulic servo-control, where it is transformedinto a high power mechanical order.

Each electromechanical actuator has at least three electromechanicalconversion members and at least three control systems, each controlsystem controlling the power supply and the operation of a respectiveelectromechanical conversion member.

Each control system has all of the elements needed for powering andcontrolling one electromechanical conversion member.

By way of example, each control systems includes means for processingelectrical control orders from the flight control device, an electronicpower device, an electronic device for controlling and monitoring theelectromechanical conversion member, sensors, and a stage forconditioning the power supply.

In order to reduce its overall size and weight, the mechanicaltransmission does not have any speed reduction member. The mechanicaltransmission may for example be a lightweight linkage comprising aconnecting rod and a crank and serving to transmit movement from anelectromechanical actuator to an input lever of a hydraulic actuator.The electromechanical actuator may thus be installed on a location thatis segregated from the hydraulic actuator, e.g. in an environment thatis not very aggressive for the electromechanical actuator, such as undera floor or in a cabin of the aircraft.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and its advantages appear in greater detail from thecontext of the following description of embodiments given by way ofillustration and with Reference to the accompanying figures, in which:

FIG. 1 shows an electrical flight control device for an aircraft;

FIG. 2 shows a first embodiment of electromechanical conversion membersof an electromechanical actuator of the invention;

FIG. 3 is a fragmentary view of an electromechanical conversion member;

FIGS. 4 and 5 show a second embodiment of an electromechanicalconversion member; and

FIG. 6 plots curves showing variation in the magnetic induction in aferromagnetic material as a function of its coercive magnetic field.

Elements present in more than one of the figures are given the samereferences in each of them.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an electrical flight control device 50 for aircraftenabling each control axis of the aircraft to be controlled. Such anelectric flight control device 50 is intended in particular forcontrolling variations in the collective and cyclic pitches of theblades of a main rotor of a rotary wing aircraft, and also variations inparticular in the collective pitch of the blades of its anti-torquerotor.

The electrical flight control device 50 comprises an electromechanicalactuator 1, a hydraulic actuator 4, and a mechanical transmission 30.The mechanical transmission 30 is a linkage comprising a connecting rod31 and a crank 32 that serves to transmit movements of theelectromechanical actuator 1 to an input lever of the hydraulic actuator4, the input lever being secured to the crank 32.

Each electromechanical actuator 1 receives an electrical control ordercoming from a flight control device 7 of the aircraft and transforms itinto a mechanical control order of low power that is transmitted via themechanical transmission 30 to the hydraulic actuator 4. The hydraulicactuator 4 can then act via the movements of its rod 9 to deliver amechanical control order of high power to a control axis of theaircraft.

Each electromechanical actuator 1 has four electromechanical conversionmembers 5 and four control systems 6, each control system 6 controllingthe electrical power supply and the operation of a respectiveelectromechanical conversion member 5. Thus, redundancy both in terms ofthe control system 6 and of the electromechanical conversion member 5serves to mitigate any type of failure that might affect theelectromechanical actuator 1, thereby guaranteeing reliable operationfor the electromechanical actuator 1.

Each control system 6 has a processor device 41 for processingelectrical control orders from the flight control device 7, anelectronic control and monitoring device 42 for controlling andmonitoring the electromechanical conversion member 5, an electronicpower device 44, sensors 45, 46, and 47, and an electrical power supplyconditioning stage 43. Furthermore, the electromechanical actuator 1 isconnected to an electrical power supply device 8 that delivers DC. Theelectronic power device 44 serves to transform the DC into a three-phaseAC voltage. Nevertheless, the electronic power device 44 could equallywell be a purely resistive lowpass circuit, e.g. filtering out highfrequencies, with the electrical power supply device 8 delivering athree-phase AC voltage directly.

FIG. 2 shows a first embodiment of the four electromechanical conversionmembers 5 of the electromechanical actuator 1. The electromechanicalconversion members 5 share an axis of rotation 2 and a transmissionshaft 3 in common. Furthermore, each electromechanical conversion member5 has a stator 10 and a rotor 20, the rotor 20 rotating about the axisof rotation 2 inside the stator 10.

FIG. 3 is a fragmentary view of an electromechanical conversion member5.

The rotor 20 secured to the transmission shaft 3 has permanent magnets23 with north poles facing each other in pairs and south poles facingeach other in pairs. This particular arrangement of the permanentmagnets 23 characterizes a flux-concentrating rotor 20. A north magneticpole 24 thus appears between two magnets 23 having their north polesfacing each other. Likewise, a south magnetic pole 25 thus appearsbetween two magnets 23 having their south poles facing each other.

The stator 10 has teeth 14, a stator yoke 18, and windings 12. Everyother tooth 14 is surrounded by a winding 12, characteristic of asingle-layer winding. Thus, each wound tooth 14 lies between two teethwithout windings.

The teeth 14, the stator yoke 18, and the magnetic poles 24, 25 are madeof ferromagnetic material, e.g. stacked laminations of iron-siliconalloy.

Furthermore, the magnetic poles 24, 25 of the rotor 20 project and areof substantially sinusoidal shape where they face the teeth 14 of thestator 10. The electromechanical conversion members 5 are thussinusoidal magnetic flux members. The airgap of each electromechanicalconversion member 5 is thus variable between a tooth 14 of a stator 10and a magnetic pole 24, 25 of a rotor 20. By way of example, this airgapvaries between a minimum value e equal to 1 mm, and a maximum value e′,equal to 2 mm. This large airgap serves advantageously to guaranteereliable operation of the electromechanical actuator 1 by reducing, oreven eliminating, any risk of any of the electromechanical conversionmembers 5 jamming as a result of a foreign body appearing in the airgap.

Furthermore, depending on the magnetic characteristics of theferromagnetic material constituting the teeth 14, the teeth 14 of thestator 10 are of dimensions such that each electromechanical conversionmember 5 operates close to its saturation limit.

A curve plotting variation of the magnetic induction of a ferromagneticmaterial as a function of its coercive magnetic field is shown in FIG. 6. The magnetic induction of the ferromagnetic material, generallyreferenced “B” and expressed in teslas (T) is plotted up the ordinateaxis while the coercive magnetic field, generally referenced “H(B)” isexpressed in amps per meter (A/m), and is plotted along the abscissaaxis.

This curve is specific for each material and has three distinctcharacteristics characterizing three different behaviors of thematerial.

A linear first portion A corresponds to the usual utilization zone for aferromagnetic material in an electromechanical conversion member 5. Thislinear first portion A is a straight line of gradient equal to themagnetic permeability of the ferromagnetic material.

A bend second portion B corresponds to the beginning of the materialsaturating. This end second portion B constitutes a saturation bend ofthe ferromagnetic material.

A linear third portion C corresponds to a zone in which theferromagnetic material is totally saturated. This linear third portion Cis a straight line of gradient equal to the magnetic permeability ofair. The slope of this linear third portion C is thus identicalregardless of the ferromagnetic material. This linear third portion Cdoes not constitute a nominal operating zone for an electromechanicalconversion member. Nevertheless, after a failure of short-circuit typein an electromechanical conversion member, the material becomesmagnetically saturated and the flux flowing through the material isdetermined by this linear third portion C.

Advantageously, each electromechanical conversion member 5 operatesclose to or at the saturation bend B of this curve plotting variation inmagnetic induction, and there is only a small increase in the magneticflux consequently in the torque of the electromechanical conversionmember 5 in the event of a short-circuit type failure. As a result, fromamong the four electromechanical conversion members 5 of theelectromechanical actuator 1, one electromechanical conversion member 5is capable of compensating for the short-circuit torque that appearsfrom an electromechanical conversion member 5 that has suffered ashort-circuit type failure. Consequently, two other electromechanicalconversion members 5 remain available for delivering the torque neededto operate the electromechanical actuator 1.

Furthermore, the short-circuit torque disappears as soon as the powersupply to the electromechanical conversion member 5 that has sufferedthis failure is interrupted by the corresponding control system 6, oncethe failure has been detected.

The dimensions of the teeth 14 are thus defined so that depending themagnetic characteristics of the ferromagnetic material constituting theteeth 14, each electromechanical conversion member 5 operates close tothe saturation bend B, or indeed in the saturation bend B. When theteeth 14 and the stator yoke 18 are made as a stack of laminations, thewidth

_(d) of the teeth 14 and the height h_(cs) of the stator yoke 18 aredefined respectively by the following formulas:

${l_{d} = {{\frac{B_{g} \cdot T_{d}}{B_{\max} \cdot K_{fe}}\mspace{14mu}{and}\mspace{14mu} h_{cs}} = \frac{\Phi_{g}}{B_{\max} \cdot L_{st} \cdot K_{st}}}};$where T_(d) is the axial length of a tooth expressed in meters (m),B_(g) is the airgap induction in nominal operation expressed in teslas(T), K_(fe) is the swelling coefficient, B_(max) is the maximum value ofthis induction expressed in teslas, Φ_(g) is the airgap flux at a rotorpole in nominal operation expressed in webers (Wb), L_(st) is the axiallength of the electromechanical conversion member expressed in meters,and K_(st) is a staking factor.

Finally, the first total number N_(p) of pairs of magnetic poles 24, 25of the rotor 20 and the second total number N_(d) of the teeth 14 of thestator 10 of an electromechanical conversion member 5 satisfy thefollowing formula:(4n−3)·N _(p) <N _(d)<(4n−1)·N _(p)where n is a positive integer. This formula corresponds in particular toan electromechanical conversion member of the brushless motor typehaving permanent magnets with flux concentration and a single-layerwinding with three-phase AC.

A second embodiment of four electromechanical conversion members 5 ofthe electromechanical actuator 1 is shown in FIGS. 4 and 5 . The fourelectromechanical conversion members 5 are assembled in pairs and eachpair of electromechanical conversion members 5 a, 5 b has a rotor 20 incommon. Furthermore, in order to obtain balanced operation, eachelectromechanical conversion member 5 a, 5 b has a stator 10 a, 10 b intwo portions, these two portions being diametrically opposite, as shownin FIG. 5 .

The common rotor 20 thus co-operates simultaneously with both stators 10a, 10 b included respectively in the two electromechanical conversionmembers 5 a, 5 b.

Furthermore, with each control system 6 powering an electromechanicalconversion member 5 with three-phase AC, each pair of electromechanicalconversion members 5 a, 5 b co-operates with the common rotor 20 toconstitute a six-phase architecture made up by assembling twothree-phase electromechanical conversion members 5 a and 5 b.

This six-phase architecture is designed for the purpose of maintainingmagnetic equilibrium. It can then be controlled by a conventionalthree-phase system, such as for example by using pulse width modulation(PWM), which control may be operated by automatic control of statorcurrents in the Park plane.

In addition, non-magnetic radial separators 11 serve to provide magneticisolation between the two stators 10 a and 10 b of this six-phasearchitecture. These non-magnetic radial separators 11 thus serve toavoid magnetic leaks appearing between the stators 10 a and 10 b, andalso to avoid any other mutual magnetic disturbance.

Naturally, the present invention may be subjected to numerous variantsas to its implementation. Although several embodiments are described, itwill readily be understood that it is not conceivable to identifyexhaustively all possible embodiments. It is naturally possible toenvisage replacing any of the means described by equivalent meanswithout going beyond the ambit of the present invention.

What is claimed is:
 1. An electromechanical actuator for electricalflight controls of an aircraft, the electromechanical actuatorcomprising: a transmission shaft having an axis of rotation; at leastthree electromechanical conversion members, each respectively providedwith a stator and a rotor secured to the transmission shaft androtatable about the axis of rotation, the stator being provided withteeth and windings, each winding surrounding at least one tooth, therotor being provided with permanent magnets each having a north magneticpole and a south magnetic pole; and at least three control systems, eachcontrol system powering and controlling a respective electromechanicalconversion member with one of the control systems being connected to asingle electromechanical conversion member and powering the singleelectromechanical conversion member with AC; wherein the teeth aredimensioned so that each electromechanical conversion member operates ata saturation bend in a curve plotting variation of a magnetic inductionof a ferromagnetic material constituting the teeth, thus enabling atleast two of the electromechanical conversion members to mitigate ashort-circuit type failure of another electromechanical conversionmember, with one of the two electromechanical conversion memberscompensating a majority of the short-circuit torque coming from a failedelectromechanical conversion member, while the other of the twoelectromechanical conversion members delivers the nominal torque neededto operate the electromechanical actuator.
 2. The electromechanicalactuator according to claim 1, wherein, when each stator is providedwith a stator yoke and each stator yoke is made up of a stack oflaminations, the width

_(d) of the teeth and the height h_(cs) of the stator yoke are definedby the following formulas:$l_{d} = {{\frac{B_{g} \cdot T_{d}}{B_{\max} \cdot K_{fe}}\mspace{14mu}{and}\mspace{14mu} h_{cs}} = \frac{\Phi_{g}}{B_{\max} \cdot L_{st} \cdot N_{st}}}$where T_(d) is the axial length of a tooth, B_(g) is the airgapinduction in nominal operation, K_(fe) is a swelling coefficient,B_(max) is a maximum value of the induction, □_(g) is an airgap flux ata pole in nominal operation, L_(st) is an axial length of anelectromechanical conversion member, and K_(st) is a stacking factor. 3.The electromechanical actuator according to claim 1, wherein eachwinding is a single-layer winding, the teeth adjacent to a toothsurrounded by the winding not being surrounded by a respective winding.4. The electromechanical actuator according to claim 1, wherein eachelectromechanical conversion member is a magnetic flux-concentratingmember.
 5. The electromechanical actuator according to claim 1, whereineach control system powers an electromechanical conversion member withpolyphase AC, and each winding is concentric, each tooth beingsurrounded by the winding in which there flows a single phase of thepolyphase AC.
 6. The electromechanical actuator according to claim 1,wherein at least two electromechanical conversion members have a rotorin common, the common rotor co-operating with the stators of the atleast two electromechanical conversion members.
 7. The electromechanicalactuator according to claim 1, wherein the magnetic poles are ofsinusoidal shape so that a sinusoidal magnetic flux flows in eachelectromechanical conversion member.
 8. The electromechanical actuatoraccording to claim 1, wherein a first total number N_(p) of pairs ofmagnetic poles and a second total number N_(d) of the teeth are suchthat:(4n−3)·N _(p) <N _(d)<(4n−1)·N _(p) where n is a positive integer. 9.The electromechanical actuator according to claim 1, wherein eachcontrol system powers one of the electromechanical conversion memberswith three-phase AC, and two electromechanical conversion members have arotor in common, the common rotor co-operating with stators of theelectromechanical conversion members the stators of the twoelectromechanical conversion members being assembled so as to co-operatewith the common rotor to constitute a six-phase architecture.
 10. Theelectromechanical actuator according to claim 9, wherein the statorsco-operating with the common rotor are magnetically isolated from eachother by non-magnetic radial separation.
 11. The electromechanicalactuator according to claim 1, wherein each stator is separated from therotor by an airgap greater than 1 mm.
 12. The electromechanical actuatoraccording to claim 11, wherein the airgap lies in a range from 1 mm to 2mm.
 13. An electrical flight control device comprising: at least oneelectromechanical actuator; at least one hydraulic actuator; and atleast one mechanical transmission, each mechanical transmission enablingthe electromechanical actuator to control a hydraulic actuator; whereineach electromechanical actuator is an actuator according to claim
 1. 14.The electrical flight control device according to claim 13, wherein eachmechanical transmission does not have a speed reduction member.
 15. Theelectrical flight control device according to claim 13, wherein eachmechanical transmission system is a linkage comprising a connecting rodand a crank.
 16. An electromechanical actuator for electrical flightcontrols of an aircraft, the electromechanical actuator comprising: atransmission shaft having an axis of rotation; at least threeelectromechanical conversion members, each respectively provided with astator and a rotor secured to the transmission shaft and rotatable aboutthe axis of rotation, the stator having teeth and windings, each windingsurrounding at least one tooth, the rotor having permanent magnets eachhaving a north magnetic pole and a south magnetic pole; pole; and atleast three control systems, each of the control systems powering andcontrolling a respective electromechanical conversion member with one ofthe control systems being connected to a single electromechanicalconversion member and powering the single electromechanical conversionmember with AC; wherein the teeth are dimensioned so that eachelectromechanical conversion member operates at the saturation bend in acurve plotting variation of a magnetic induction of a ferromagneticmaterial constituting the teeth, thus enabling at least two of theelectromechanical conversion members to mitigate a short-circuit typefailure of another electromechanical conversion member, with one of thetwo electromechanical conversion members compensating the majority ofthe short-circuit torque coming from a failed electromechanicalconversion member, while the other of the two electromechanicalconversion members delivers the nominal torque needed to operate theelectromechanical actuator; and wherein, each stator is provided with astator yoke and each stator yoke is made up of a stack of laminations,wherein the permanent magnets are arranged in such a manner as toprovide flux concentration in the airgap, thereby maximizing the torqueof the electromechanical conversion member.
 17. The electromechanicalactuator according to claim 16, the width

_(d) of the teeth and the height h_(cs) of the stator yoke are definedby the following formulas:$l_{d} = {{\frac{B_{g} \cdot T_{d}}{B_{\max} \cdot K_{fe}}\mspace{14mu}{and}\mspace{14mu} h_{cs}} = \frac{\Phi_{g}}{B_{\max} \cdot L_{st} \cdot K_{st}}}$where T_(d) is the axial length of a tooth, B_(g) is the airgapinduction in nominal operation, K_(fe) is a swelling coefficient,B_(max) is a maximum value of the induction, □_(g) is an airgap flux ata pole in nominal operation, L_(st) is an axial length of anelectromechanical conversion member, and K_(st) is a stacking factor.18. The electromechanical actuator according to claim 16, wherein eachelectromechanical conversion member is a magnetic flux-concentratingmember and wherein each control system powers an electromechanicalconversion member with polyphase AC, and each winding is concentric,each tooth being surrounded by the winding in which there flows a singlephase of the polyphase AC.
 19. The electromechanical actuator accordingto claim 16, wherein each control system powers one of theelectromechanical conversion members with three-phase AC, and twoelectromechanical conversion members have a rotor in common, the commonrotor co-operating with stators of the electromechanical conversionmembers the stators of the two electromechanical conversion membersbeing assembled so as to co-operate with the common rotor to constitutea six-phase architecture.
 20. The electromechanical actuator accordingto claim 16, wherein a first total number N_(p) of pairs of magneticpoles and a second total number N_(d) of the teeth are such that:(4n−3)·N _(p) <N _(d)<(4n−1)·N _(p) where n is a positive integer.